The present invention relates to a plate lockup apparatus provided in a plate cylinder of a printing press for fixing leading and trailing sides of a plate to be wound on a circumferential surface of the plate cylinder.
Generally, a gap having a substantially rectangular section is formed in the circumferential surface of the plate cylinder of each of various types of printing presses to extend for almost an entire length of the cylinder. A plate lockup apparatus consisting of a leading-side plate lockup device for gripping the leading end of a plate and a trailing-side plate lockup device for gripping the trailing end of the plate gripped by the leading-side plate lockup device and wound on the circumferential surface of the cylinder extends in the gap in the axial direction of the cylinder by being supported on the bottom surface of the gap.
Conventionally, of the plate lockup apparatuses of this type, a spring closed plate lockup apparatus has a structure as disclosed in, e.g., Japanese Patent Laid-Open No. 3-65343 proposed by the present applicant. More specifically, each of the leading- and trailing-side plate lockup devices of this plate lockup apparatus has an elongated lockup table extending in the axial direction of the cylinder, and gripper plates pivotally supported by the lockup table and opened and closed by rotation of the lockup table to grip and release the plate with cooperation with the lockup table. Leading- and trailing-side cam shafts each having a plurality of cams to engage with the corresponding gripper plates extend in the vicinity of the gripper plates in the axial direction of the cylinder. In the spring closed plate lockup apparatus having the above structure, compression coil springs are interposed between the gripper plates and each lockup table to bias the gripper plates in a closing direction toward the corresponding lockup table.
In the conventional plate lockup apparatus having the above structure, when the plate is to be gripped, the leading-side cam shaft is pivoted manually or by a driving unit to open the gripper plates by the function of the cams against the elastic force of the compression coil springs, and one end of the plate is inserted between the gripper surfaces of the gripper plates and the lockup table. Then, the cam shaft is pivoted in the reverse direction, so that the gripper plates are released from the function of the cams and are closed by the elastic force of the compression coil springs, thus gripping one end of the plate. When the plate cylinder is rotated by almost one revolution, the plate is wound on the circumferential surface of the cylinder. The trailing end of the plate is gripped by the trailing-side plate lockup device in the same manner as in the leading end of the plate. When the trailing-side cam shaft is further pivoted, the trailing-side plate lockup device is moved in the circumferential direction of the plate, thus tightening the plate to be brought into tight contact with the circumferential surface of the cylinder.
However, in such a conventional plate lockup apparatus, since the plate gripping operation by the plate lockup devices and the plate tightening operation are performed by the spring force, the plate cannot sometimes be sufficiently gripped or mounted depending on its thickness or material. In addition, the structure of the apparatus is complicated. Also, the trailing end of the plate must be bent almost at 90.degree. to the circumferential surface of the plate so that the plate is gripped by the trailing-side plate lockup device. For this purpose, a special plate bending machine must be used.